1. Field of the Invention
The present invention generally relates to transmission apparatuses, order wire transmission systems and order wire monitoring methods, and more particularly to a transmission apparatus which includes functions for transmitting and receiving multiplexed signals in various kinds of networks and functions for transmitting and receiving order wire signals, an order wire transmission system which uses such a transmission apparatus, and an order wire monitoring method for monitoring quality and the like of an order wire line.
2. Description of the Related Art
FIG. 1 is a system block diagram generally showing an order wire transmission system. In FIG. 1, transmission apparatuses A and B are connected via a radio or cable line, transmission apparatuses C and D are connected via a radio or cable line, and transmission apparatuses E and F are connected via a radio or cable line. Each of the transmission apparatuses A through F includes functions for transmitting and receiving a main signal with another transmission apparatus, and functions for transmitting and receiving an order wire signal. The transmission apparatuses B, C and E are located within a single office S, and the order wire signal is branched and combined in the office S.
FIG. 2 is a system block diagram for explaining an important part of a conventional transmission apparatus. In FIG. 2, a transmission apparatus 71 corresponds to the transmission apparatus B shown in FIG. 1. The transmission apparatus 71 includes an optical or radio transmitter and receiver section 72 transmits and receives an optical signal or a radio signal, a multiplexing and demultiplexing section 73, and an order wire section 74. The order wire section 74 includes a codec section 75, an analog branching and combining section 76, an office Dual Tone Multi Frequency (DTMF) transmitting and retrieving section 77, and a 2-wire/4-wire (2W/4W) converter 78. A telephone set 79 is connected to the 2W/4W converter 78. The transmission apparatuses C and D have the same construction as the transmission apparatus B (71), and are connected to the transmission apparatus B (71). In FIG. 2, the illustration of transmission paths for the multiplexed signals exchanged among the transmission apparatuses B, C and D is omitted.
The multiplexing and demultiplexing section 73 carries out a multiplexing process and a demultiplexing process in conformance with a multiplexing system such as Plesiochronous Digital Hierarchy (PDH), Synchronous Digital Hierarchy (SDH) and the like. For example, the order wire signals are multiplexed in specific time slots in the case of the PDH, and the order wire signals are multiplexed by section overhead bytes E1 and E2 in the case of the SDH. Accordingly, the multiplexing and demultiplexing section 73 is constructed to multiplex and demultiplex the order wire signals depending on the multiplexing system employed.
When connecting to an optical line, the optical or radio transmitter and receiver section 72 has optical-to-electrical converting functions and optical wavelength multiplexing and demultiplexing functions. On the other hand, when connecting to a radio line, the optical or radio transmitter and receiver section 72 has high-frequency transmitting and receiving functions corresponding to transmission frequency or modulation technique employed. In addition, when connecting to a cable line for exchanging electrical signals, the optical or radio transmitter and receiver section 72 has functions for transmitting and receiving digital multiplexed signals.
In the order wire section 74, the codec section 75 carries out coding and decoding, including analog-to-digital conversion. The office DTMF transmitting and retrieving section 77 transmits, receives and identifies a DTMF signal used by push-button type telephone sets. The 2W/4W converter 78 carries out a 2-wire-to-4-wire (2W/4W) conversion. The digital received order wire signals which are demultiplexed by the multiplexing and demultiplexing section 73 are converted into analog signals by the codec section 75, and are branched into three by the analog branching and combining section 76. The office DTMF transmitting and retrieving section 77 judges whether or not the received DTMF signal specifies the transmission apparatus 71 to which the office DTMF transmitting and retrieving section 77 belongs. The office DTMF transmitting and retrieving section 77 also includes functions for transmitting a DTMF signal which specifies another transmission apparatus, such as the transmission apparatus C or E.
When making a call using the telephone set 79, audio signals are input to the codec section 75 via the 2W/4W converter 78 and the analog branching and combining section 76, and are converted into digital signals. The digital signals are input to the multiplexing and demultiplexing section 73, and are multiplexed, as order wire signals, to a main signal, in conformance with the multiplexing system employed.
For example, when communicating between the transmission apparatuses A and F shown in FIG. 1 via an order wire line, the branching and combining of the order wire signals are carried out via the analog branching and combining section 76 in the intermediate transmission apparatus B as shown in FIG. 2. In addition, in the transmission apparatuses B and E which are located between the transmission apparatuses A and F, the telephone sets are put into the on-hook state. For example, if the telephone set 79 of the transmission apparatus B, which is located between the transmission apparatuses A and F, is put into the off-hook state, the communication using the order wire signals cannot be made between the transmission apparatuses A and F. But in this state, a communication using the order wire signals is possible with the transmission apparatus A or the transmission apparatus F using this telephone set 79.
In a system which transmits multiplexed signals by connecting a plurality of transmission apparatuses by a line such as the cable line, optical line and radio line, the order wire line corresponding to one channel is prepared for making a prearranged communication between the transmission apparatuses. This order wire line is shared by each of the transmission apparatuses, so as to enable the prearranged communication between arbitrary transmission apparatuses. In such a system, even when the multiplexed main signal can be transmitted and received, a connection error may exist in one transmission apparatus with respect to the order wire line, in which case the order wire line cannot be connected between the transmission apparatuses which are located on both ends of this one transmission apparatus.
Accordingly, in a case of a failure where only the order wire line cannot be connected, it is necessary to send a maintenance or service person to each transmission apparatus, to make a prearranged communication test between the transmission apparatuses, and to find the transmission apparatus which cannot make the prearranged communication, so that a restoration process can be carried out with respect to the failure. Normally, however, the transmission apparatuses are scattered, and two transmission apparatuses are separated by a distance on the order of several tens of km or greater. For this reason, it takes considerable time and effort on the part of the maintenance or service persons, due to the need to simultaneously or successively send the maintenance or service person to each transmission apparatus and to carry out the prearranged communication test.
In addition, the order wire signals which are demultiplexed from the multiplexed main signal are converted into analog signals by the codec section 75, branched by the analog branching and combining section 76 and distributed to the telephone set and the adjacent transmission apparatuses. Moreover, the analog order wire signals are combined by the analog branching and combining section 76, and converted into digital signal by the codec section 75. In other words, the digital-to-analog conversion is carried out every time the order wire signals are branched, and the analog-to-digital conversion is carried out every time the order wire signals are combined. As a result, quantization errors are accumulated by the conversions which are carried out repeatedly, and a failure may be generated in the prearranged communication due to this quantization error accumulation. When such a failure occurs, it is also necessary to send the maintenance or service person to each transmission apparatus and to successively carry out the prearranged communication test via the order wire line between the transmission apparatuses, similarly as described above, in order to find the cause of the failure and correct the failure. Consequently, it takes considerable time and effort on the part of the maintenance or service persons to find and correct the failure.